Medical ultrasound images are typically produced by generating an ultrasonic sound wave traveling in a known direction and observing the echoes created when the sound wave bounces off of boundaries between regions of differing density in the body. For any given direction, the image pixels are generated by plotting a dot whose brightness is proportional to the echo's amplitude at a coordinate whose location is a function of the time after a short pulse is send in the direction in question.
Medical ultrasound images are often very "noisy". In addition to the boundaries between vessels and organs, the images include a significant amount of speckling that make the detection of the boundaries of interest difficult. One method for reducing the speckling involves combining two or more frames in a time sequence. Unfortunately, respiration and involuntary movements such as heart beating cause the various organs to move relative to the ultrasound probe. This motion causes the boundaries of the organs to appear blurred when two or more frames are averaged.
Sequences of ultrasound images may also be used, in principle, to provide an analysis of the motion of the various tissue structures .i In those cases in which the analysis indicates that the boundaries have not moved significantly, frame averaging may be utilized to improve the clarity of the images. In addition, motion analysis is useful in medical diagnosis.
Unfortunately, the lack of clarity in the images makes it difficult to provide reliable motion analysis. The techniques used in the prior art to perform motion analysis in ultrasound imaging may be grouped into one-dimensional and two-dimensional techniques. In one-dimensional measurements the echoes acquired along the ultrasoud beam direction line at different times are compared to detect the motion of a tissue or fluid. These techniques involve computation of the correlation of successive scans in the same direction. The measured correlation decreases with increasing motion, thereby providing a measurement of the amount of motion. Unfortunately, such techniques are limited to measuring the axial component, i.e., along the beam direction, of the motion.
The two-dimensional motion analysis techniques used in the prior art can be divided into two groups referred to as block-matching and optical flow in the following discussion. The block matching techniques attempt to match a block in the first frame to a block in the second frame by computing the correlation of the block in the first frame with each block of the same size in a defined search region of the second frame. The block in the second frame having the highest correlation is assumed to be the matching block, provided the correlation is above a threshold. The displacement of the two blocks provides an estimate of the motion of the corresponding region in the first frame. This technique is limited to the detection of linear translation of the block in question. If the motion is more complicated the results are less useful, since different portions of the block will have different velocities. This type of more complicated motion is often encountered in medical imaging because of muscle contractions..
In the non-linear motion cases, optical flow methods have been found to be more reliable. These models assume that the brightness of a particular point in a pattern is constant as the pattern moves. Unfortunately, the computational workload inherent in prior art optical flow computations is too high for implementation in routine ultrasoud imaging. Approximations to the optical flow method requiring lower computational resources have been proposed; however, the results from these techniques are very sensitive to noise.
Broadly, it is the object of the present invention to provide an improved ultrasound imaging system.
It is a further object of the present invention to provide an ultrasound imaging system that can measure the motion of various points in a sequence of ultrasoud images.
It is a still further object of the present invention to provide an ultrasoud imaging system that can combine successive frames in a sequence to provide improved image quality even when the boundaries that are being imaged are in motion.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.